Image processing apparatus and control method thereof

ABSTRACT

The invention distinguishes not only character areas and halftone dot areas in an image, but also makes highly accurate decisions concerning characters present in halftone dot areas. A decision signal generating unit generates a decision signal having luminance as a main component from a signal indicating inputted image data, and supplies the decision signal to a character decision unit, a halftone dot decision unit, and a character-in-halftone dot decision unit. The character decision unit generates data indicating whether a pixel of interest is inside a character image area. The halftone dot decision unit generates data indicating whether a pixel of interest is inside a halftone dot area. The character-in-halftone dot decision unit generates data indicating whether there is a character image inside the halftone dot area. Based on these three signals, an attribute flag generating unit generates attribute data of the pixel of interest.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to techniques by which an attribute ofeach pixel in an image is determined.

2. Description of the Related Art

Apparatuses that extract character edges using a differential filter orthe like then classify characters and elements other than characters toexecute adaptive processing are known as image processing apparatusesthat execute optimal image processing according to imagecharacteristics. Furthermore, apparatuses that take halftone dots ofprinting as isolation amounts then extract the halftone dots byaccumulating the isolation amounts within a predetermined area andclassify characters and halftone dots to execute adaptive processing arecommon (for example, Japanese Patent Laid-Open No. H11-85978.

Furthermore, methods have been proposed that identify not onlycharacters and halftone dots but also extract characters within halftonedots as low frequency edges (for example, Japanese Patent Laid-Open No.2006-5806.

However, although there is a high extraction capability (accuracy) forhalftone dots that express characters on a white background and printedphotos and the like with the conventional technique described inJapanese Patent Laid-Open No. H11-85978, the extraction capability forcharacters and line drawings within halftone dots is low. The extractioncapability for characters in halftone dots is improved when thecharacter extraction capability is enhanced as an attempt to extractcharacters in halftone dots, but at the same time image edges inhalftone dots may end up being misidentified, and as a result characterprocessing is executed also on photo image areas of printed materials,which incurs reduced image quality.

To solve this, it is necessary to separately identify characters on awhite background and characters in halftone dots and execute suitableprocessing on these respective characters.

The conventional technique described in Japanese Patent Laid-Open No.2006-5806 was proposed to solve this issue, but there are several typesof halftone dots and it does not satisfactorily separate halftone dotsand characters with good accuracy. Therefore, depending on the halftonedot document, there is a problem of moire occurring.

SUMMARY OF THE INVENTION

The present invention has been devised in light of these problems andprovides technology that distinguishes not only character areas andhalftone dot areas in an image, but also makes highly accurate decisionsconcerning characters present in halftone dot areas. The presentinvention provides technology that focuses on characters present inhalftone dot areas and decides whether or not there are characters inthe areas with high accuracy. Further still, the present inventionprovides technology that uses the decision results to adaptively switchimage processing in response to attributes, thus obtaining high qualityprinting results in which occurrences of moire are suppressed.

According to an aspect of the present invention, an image processingapparatus in which an attribute of each pixel constituting image data isdecided includes: a generation unit adapted to generate a decisionsignal having luminance as a main component from the image data; acharacter decision unit adapted to, based on the decision signalgenerated by the generation unit, generate inside edge data indicatingwhether a pixel of interest is positioned on an image edge of a lowluminance side at which the luminance changes from low luminance to highluminance, outside edge data indicating whether the pixel of interest ispositioned on an image edge of a high luminance side at which theluminance changes from high luminance to low luminance, and accumulationdata of inside edge data and outside edge data of a predetermined numberof pixels positioned around the pixel of interest, and decide whetherthe pixel of interest is in a character area based on the inside edgedata, the outside edge data, and the accumulation data; a halftone dotdecision unit adapted to, based on the decision signal generated by thegeneration unit, generate inside edge data indicating whether the pixelof interest is positioned on an image edge of a low luminance side atwhich the luminance changes from low luminance to high luminance andoutside edge data indicating whether the pixel of interest is positionedon an image edge of a high luminance side at which the luminance changesfrom high luminance to low luminance, calculate an isolation degree foreach of multiple patterns by matching isolation patterns of differentsizes against the inside edge data and outside edge data of apredetermined number of pixels around and including the pixel ofinterest, and decide whether the pixel of interest is in a halftone dotarea based on the calculated isolation degrees; an adaptive filter unitadapted to decide whether there is continuity in low luminance pixels ofa preset plurality of directions based on the decision signal generatedby the generation unit, and allow the decision signal to pass when it isdecided that there is continuity and carry out a low pass filter processdefined by a preset frequency when it is decided that there is nocontinuity; a character-in-halftone dot decision unit adapted to, basedon the decision signal generated by the adaptive filter unit, generateinside edge data indicating whether the pixel of interest is positionedon an image edge of a low luminance side at which the luminance changesfrom low luminance to high luminance and decide whether the pixel ofinterest is a character area in a halftone dot area based on thegenerated inside edge data; and an attribute data generation unitadapted to generate attribute data of the pixel of interest based ondecision results of the character decision unit, the halftone dotdecision unit, and the character-in-halftone dot decision unit.

The configuration of the present invention distinguishes not onlycharacter areas and halftone dot areas in an image, but can also makehighly accurate decisions concerning characters present in halftone dotareas. Furthermore, it uses the decision results to adaptively switchimage processing in response to attributes, thus making it possible toobtain high quality printing results in which occurrences of moire arecontrolled.

Further features of the present invention will be apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is a block configuration diagram of an exemplary copier.

FIG. 2 is a block configuration diagram of an image area separationprocessing unit shown in FIG. 1.

FIG. 3 is a block configuration diagram of a character decision unitshown in FIG. 2.

FIG. 4 is a block configuration diagram of a halftone dot decision unitshown in FIG. 2.

FIGS. 5A to 5F show various processing signals of the character decisionunit.

FIG. 6 shows a data structure of an LUT of the character decision unit.

FIGS. 7A to 7C are diagrams illustrating processing of the characterdecision unit.

FIGS. 8A to 8C are diagrams illustrating processing of the halftone dotdecision unit.

FIGS. 9A and 9B are diagrams illustrating a pattern matching process ofthe halftone dot decision unit.

FIGS. 10A to 10D show examples of a pattern matching process of thehalftone dot decision unit.

FIGS. 11A to 11C show examples of a pattern matching process of thehalftone dot decision unit.

FIG. 12 shows an example of a specific configuration pertaining tohalftone dot decisions by the halftone dot decision unit.

FIGS. 13A to 13C show one example of a process of the halftone dotdecision unit.

FIGS. 14A to 14C show one example of a process of the halftone dotdecision unit.

FIGS. 15A to 15C show one example of a process of the halftone dotdecision unit.

FIG. 16 is a block configuration diagram of a character-in-halftone dotdecision unit shown in FIG. 2.

FIG. 17 is a block configuration diagram of an adaptive smoothing unitshown in FIG. 16.

FIGS. 18A to 18D show examples of line portion detection patterns of avertical line, horizontal line, and slanted line detection unit in FIG.17.

FIGS. 19A to 19C are diagrams illustrating detection examples of lineportion detection patterns of the vertical line, horizontal line, andslanted line detection unit.

FIGS. 20A to 20C are diagrams illustrating detection examples of lineportion detection patterns of the vertical line, horizontal line, andslanted line detection unit.

FIG. 21 shows space filter frequency characteristics used by theadaptive smoothing unit.

FIGS. 22A to 22C are diagrams illustrating specific processing examplesof the adaptive smoothing unit.

FIG. 23 is a block configuration diagram of an exemplary image areaseparation processing unit.

FIG. 24 is a block configuration diagram of an isolation amount decisionunit.

FIGS. 25A and 25B show examples of patterns of the isolation amountdecision unit.

FIG. 26 is a block configuration diagram of the character-in-halftonedot decision unit.

FIG. 27 is a block configuration diagram of an output image processingunit shown in FIG. 1.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present invention are described in detailsbelow with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a block configuration diagram of an image processing apparatusaccording to the present embodiment. In the present embodiment,description is given of an example in which a copier (multifunctionperipheral) also functions as a network printer. Below, a description isgiven of processing conducted by each unit in FIG. 1.

An operator loads a document to be copied into an ADF (auto documentfeeder) unit (not shown) of a scanner unit 101 and gives instruction onan operation unit (not shown) for commencement of copying. As a result,the ADF unit carries the document page by page onto a platen glassprovided in the scanner unit 101 and reading is carried out. Using anRGB 3-line CCD for reading color images, the scanner unit 101 reads adocument image and transfers this to an input image processing unit 102as 8-bit RGB (256 gradations) digital data. The input image processingunit 102 carries out commonly known image processing techniques, such asshading correction, CCD interline correction, and color correction, onthe RGB color image data sent from the scanner unit 101.

An image area separation processing unit 103 carries out image areaseparation processing on image processed color image signals output fromthe input image processing unit 102. The image area separationprocessing unit 103 detects image characteristics of photo (naturalimage) areas, character areas, and halftone dot areas for each pixel ofthe input image, then generates flag data indicating attributes of eachimage area and outputs this data to a flag memory 106.

Based on the flag data generated by the image area separation processingunit 103, an input image processing unit 104 carries out appropriateimage processing for each image area and stores a result thereof in animage memory 105. For example, processing can be carried out ofemphasizing high frequency components of the images for character areasto emphasize the sharpness of the characters, and so-called low passfilter processing for halftone dot areas to remove moire components thatare peculiar to digital images. The switching of these processes iscarried out on a per-pixel basis according to attribute flag datagenerated by the image area separation processing unit 103.

When one page of data is stored in the image memory 105 and the flagmemory 106, these undergo compression coding in a data compression unit109 and are then stored in a storage unit 110, respectively. Thisprocessing is carried out for all the documents loaded in the scannerunit 101. It should be noted that the data compression unit 109 performsencoding on the flag data using a lossless compression technique. JPEG,which is a lossy encoding technique, is used for the image data, but alossless compression technique may be used.

Document reading processing is carried out and the storage unit 110accumulates the image data and flag data of each document that is read.During this accumulation process, a data decompression unit 112 readsout the image data and flag data that have undergone compression codingby the storage unit 110 and carries out a decompression (decoding)process. At this time, resolution conversion of the image data is alsocarried out as necessary by a resolution conversion unit 113.

Decompressed image data is stored in an image memory 114 anddecompressed flag data is stored in a flag memory 115. An output imageprocessing unit 116 converts the image data that has been stored in theimage memory 114 into recording color components C, M, Y, and Bk.Furthermore, the output image processing unit 116 carries out a processof reading out attribute information for each pixel from the flag memory115 to generate image data for printing suited to each image area andoutputs to a printer engine 117.

The foregoing description concerned a copying process but the apparatusof the present embodiment is connected to an external communicationsroute (network) 119 and also functions as a network printer.

In this case, print data that has been received from an external PC orthe like via a communications I/F 118 is supplied to an interpreter 108and an assistive storage unit 111. The interpreter 108 interprets PDLcommands of the received print data and converts these to internaldrawing commands (intermediate data). A RIP (Raster Image Processor) 107carries out a drawing process based on these drawing commands andoutputs the result thereof to the image memory 105 and outputs flag datato the flag memory 106. PDL format print data defines characters, photos(natural images), and halftone dots and the like as print commands andtherefore flag data can be obtained easily when drawing.

The foregoing was a description of a printing process according to thepresent embodiment. Next, more detailed description is given concerningthe image area separation processing unit 103 according to the presentembodiment.

FIG. 2 is a block configuration diagram of the image area separationprocessing unit 103 shown in FIG. 1.

A decision signal generating unit 1002 inputs an input signal 1001 fromthe input image processing unit 102 and generates a decision signal(data) for deciding attributes on a per-pixel basis. For example, whenthe input signals are RGB signals (8 bits each), a grayscale signal (8bits) is generated. At this time, it is possible that only the G channelof RGB channels is drawn out and it is also possible that it isdetermined by a calculation such as (R+2×G+B)/4 or the like. Dependingon the circumstances, the RGB color space may be converted to a Labcolor space then the L data thereof may be used. That is to say, thenumber of input signal channels and the number of bits are not limitedto these. Also, in regard to decision signal generating methods, channelnumbers, and bit numbers, the foregoing is merely an example.

The decision signals (luminance data) generated by the decision signalgenerating unit 1002 are supplied to a character decision unit 1003, ahalftone dot decision unit 1004, and a character-in-halftone dotdecision unit 1005, where character deciding, halftone dot deciding, andcharacter-in-halftone dot deciding are respectively executed. Anattribute flag generating unit 1006 performs a comparison calculation ondecision results from the decision units 1003 to 1005 and generatesattribute flags. For the present embodiment, description is givenconcerning generating a character flag 1007, a halftone dot flag 1008, acharacter-in-halftone dot flag 1009, and a photo image flag 1010. Basedon these attribute flags, optimal image processing can be executedaccording to characteristics of the image contained in the documentimage. Below, detailed description is given concerning the characterdecision unit 1003, the halftone dot decision unit 1004, and thecharacter-in-halftone dot decision unit 1005.

[Description of Character Decision Unit 1003]

First, the character decision unit 1003 is described using FIG. 3. FIG.3 is a block configuration diagram of the character decision unit 1003according to the present embodiment.

An edge emphasizing unit 1102 carries out an edge emphasizing process onthe decision signals from the decision signal generating unit 1002 andoutputs edge emphasized data. In this edge emphasizing process, adigital filtering process is carried out by which predeterminedfrequency components of luminance data are emphasized and extracted. Atypical example of this is a secondary differential filter or the likesuch as a Laplacian. Therefore, the edge emphasizing unit 1102 has abuilt-in buffer memory for storing multiple pixels.

The edge emphasized signals output from the edge emphasizing unit 1102are input to a threshold decision unit 1103 and a threshold decisionunit 1104 where secondary differential filter processing is executed. Apositive value threshold is set in the threshold decision unit 1103 anda negative value threshold is set in the threshold decision unit 1104.

When using a secondary differential filter, the values after filteringhave positive or negative signs. A case in which the image signals areluminance signals as in the present embodiment is described using FIGS.7A to 7C. In FIG. 7A, an edge boundary area 1501 indicates a portion ofa black character (character edge) on a white background 1503. Numeral1502 indicates a character inside area. Viewing a section 1510 of theedge boundary area 1501 as a signal level gives an image signal 1504shown in FIG. 7B. The character inside area 1502 is a dark area andtherefore the signal level is low (1505). The background 1503 is abright area and therefore the signal level is high (1506). The imagesignal 1504 subjected to digital filtering process using a secondarydifferential filter becomes an edge extraction signal 1507 shown in FIG.7C. The value of the edge extraction signal 1507 on the character insidearea 1502 side takes a positive value and that on the background 1503side takes a negative value. Thus, an inside edge signal (a signalindicating that a pixel of interest is inside a character edge (1508))is output from the threshold decision unit 1103 when the positivethreshold is exceeded. On the other hand, an outside edge signal (asignal indicating that a pixel of interest is outside a character edge(1509)) is output from the threshold decision unit 1104 when the signalvalue is below the negative threshold.

The inside edge signal output from the threshold decision unit 1103 issupplied to an area accumulating unit 1105 and the outside edge signaloutput from the threshold decision unit 1104 is supplied to an areaaccumulating unit 1106. For example, the area accumulating units 1105and 1106 add decision signals of pixels of a 3×3 area around andincluding pixels of interest constituted of a 3×3 size. In this case,the values output from the area accumulating units 1105 and 1106 are ina range of 0 to 9.

Threshold decision units 1107 and 1108 output decision results bycomparing output results of the area accumulating units 1105 and 1106with a threshold. For example, when a value of “2” is set for thethresholds respectively, the threshold decision unit 1107 outputs a “1”decision signal “when there are two or more pixels determined to beinside the edge within the surrounding 3×3 area.” Similarly, thethreshold decision unit 1108 outputs a “1” decision signal “when thereare two or more pixels determined to be outside the edge within thesurrounding 3×3 area.” An example of the foregoing decision processingis described using FIGS. 5A to 5F. FIG. 5A shows a character edgeportion 1301 on a white background.

An area 1302 in FIG. 5B shows a result (outside edge) obtained by theinput signal having undergone edge emphasizing by the edge emphasizingunit 1102 and a threshold decision by the threshold decision unit 1104.As shown in the diagram, the area 1302 indicates an outside area of thecharacter edge.

An area 1304 in FIG. 5C shows an area accumulation result of the outsideedge obtained by processing the outside edge area 1302 with the areaaccumulating unit 1106 and performing a threshold decision with thethreshold decision unit 1108. As shown in the diagram, it is evidentthat the area 1304 is a result of the outside edge area 1302 beingexpanded.

An area 1303 in FIG. 5D shows the inside edge area as a result obtainedby processing the input signal 1301 with the edge emphasizing unit 1102and performing a threshold decision with the threshold decision unit1103. As exemplified in the diagram, the inside edge area 1303 indicatesan inside area of the character edge 1301.

An area 1305 in FIG. 5E shows an area accumulation of the inside edgearea obtained by processing the inside edge signal 1303 with the areaaccumulating unit 1105 and a performing a threshold decision with thethreshold decision unit 1107. As exemplified in the diagram, it isevident that the area 1305 is a result of the inside edge signal 1303being expanded.

An LUT (look-up table) 1109 inputs signals (3 bits) of the thresholddecision units 1103, 1107, and 1108 and generates signals indicatingwhether or not these are character edges. An area 1306 in FIG. 5F showsa character edge area.

The LUT 1109 performs a role in outputting decision results inaccordance with logic that is determined based on output results of therespective threshold decisions. For example, there are the followingconditions.

Pixel of interest is inside edge.→1

Other than above, two or more outside edges present in 5×5 area aroundpixels of interest.→0 Other than above, two or more inside edges presentin 5×5 area around pixels of interest.→1 None of the above apply.→0

To achieve the above, the LUT 1109 should hold a table such as the onein FIG. 6. As exemplified in the diagram, character edge decisions aredecided in response to results of an inputted inside edge decision, anarea accumulation decision of the inside edge decision, and an areaaccumulation decision of the outside edge decision. By performingdecisions in the above manner, character edges can be suitablyextracted. A character signal 1110 is generated based on the LUT and thedecisions of the decision units.

[Description of Halftone Dot Decision Unit 1004]

Next, description is given of the halftone dot decision unit 1004according to the present embodiment. FIG. 4 is a block configurationdiagram of the halftone dot decision unit 1004 according to the presentembodiment.

An edge emphasizing unit 1202 inputs a decision signal (luminance datain the present embodiment) from the decision signal generating unit 1002and carries out an edge emphasizing process. Here, a digital filteringprocess is carried out by which predetermined frequency components ofthe image data are emphasized and extracted. A typical example of thisis a secondary differential filter or the like such a Laplacian.

The edge emphasized signals output from the edge emphasizing unit 1202are supplied to a threshold decision unit 1203 and a threshold decisionunit 1204 where secondary differential filter processing is carried out.A positive value threshold is set in the threshold decision unit 1203and a negative value threshold is set in the threshold decision unit1204.

When using a secondary differential filter, the filtered signal valueshave positive or negative signs, and these are shown in FIGS. 8A to 8Cfor a case in which the image signals are luminance signals as in thepresent embodiment. When interest is given to a single halftone dot, anedge boundary area 1601 of that halftone dot indicates a portion (ahalftone dot edge) of the halftone dot in a white background 1603.Numeral 1602 indicates a halftone dot. Viewing a section 1610 of thehalftone dot edge boundary area 1601 as a signal level gives an imagesignal 1604. The halftone dot 1602 is a dark area and therefore thesignal level is low (1605). The background 1603 is a bright area andtherefore the signal level is high (1606). The image signal 1604subjected to digital filtering process using a secondary differentialfilter becomes a halftone dot edge extraction signal 1607. The value ofthe halftone dot edge extraction signal 1607 on the halftone dot 1602side takes a positive value 1608 and that on the background 1603 sidetakes a negative value 1609. Thus, an inside edge signal is output fromthe threshold decision unit 1203 that is decided when the positivethreshold is exceeded and an outside edge signal is output from thethreshold decision unit 1204 that is decided when the signal value isbelow the negative threshold.

The inside edge signal output from the threshold decision unit 1203 isinput to an isolation amount decision unit 1205. The outside edge signaloutput from the threshold decision unit 1204 is input to an isolationamount decision unit 1206.

The isolation amount decision unit 1205 carries out a pattern matchingprocess on the inside edge signals from the threshold decision unit1203. Halftone dot documents range from low screen frequencies to highscreen frequencies and therefore the sizes and intervals of the halftonedots vary depending on the document. For this reason, pattern matchingis carried out using a plurality of patterns so as to be able to detecthalftone dots of any kind of screen frequency. For halftone dots of alow screen frequency, pattern matching is carried out using largepatterns to detect whether or not there is a halftone dot. For halftonedots of a high screen frequency, pattern matching is carried out usingsmall patterns to detect whether or not there is a halftone dot.Furthermore, the shape of halftone dots changes depending on theirluminance, and therefore levels are applied in the matching so as to beable to handle this.

An example of pattern matching is described using FIGS. 9A and 9B. FIG.9A shows an example of a result from the threshold decision unit 1203.Description is given of an example in which pattern matching isperformed for 4×4 pixels 1700, and therefore an inside edge signal 1701of the 4×4 area is extracted. Therein, pixels output by the thresholddecision unit 1203 as HIGH inside edge signals are 4 pixels (blackpixels) of an area 1702 shown in the diagram. The remaining pixels arepixels of LOW inside edge signals (1703). And the pixel of interest is apixel 1701.

Next, an example of a same size pattern is shown in FIG. 9B. In apattern 1710 of this diagram, a black pixel 1712 is a pixel for apattern HIGH. A white pixel 1713 is a pixel for a pattern LOW. Aslanting line pixel 1714 is a pixel in which either is acceptable(“don't care,” hereinafter referred to as “DC”). With this as a basicpattern, decision levels are adjusted by applying levels to the extentof pattern matching for each of the black pixel 1712 and the white pixel1713. And the pixel of interest is a pixel 1711.

FIGS. 10A to 10D show an example of pattern matching between the insideedge signal 1700, which is a result of the threshold decision, and thepattern 1710 of the isolation amount decision.

In the isolation amount decision pattern 1710, when three or more pixelsof a pattern HIGH (1712) of 4 pixels are matching, the pattern HIGH isvalid. When 6 or more pixels of a pattern LOW (1713) of 8 pixels arematching, the pattern LOW is valid. The isolation amount decision resultof the pixel of interest 1801 is HIGH only when both the pattern HIGHand the pattern LOW are valid.

As a result of the foregoing, in the case of an inside edge signal 1810of FIG. 10A, including a pixel of interest 1801, since 4 pixels arepattern HIGH and 8 pixels are pattern LOW, both are valid and as aresult the isolation amount decision signal becomes HIGH.

In the case of an inside edge signal 1820 of FIG. 10B, since 4 pixelsare pattern HIGH and 7 pixels are pattern LOW, both are valid and as aresult the isolation amount decision signal becomes HIGH. It should benoted that a pixel 1821 is “don't care,” and therefore does notinfluence the decision.

In the case of an inside edge signal 1830 of FIG. 10C, since 3 pixelsare pattern HIGH and 8 pixels are pattern LOW, both are valid and as aresult the isolation amount decision signal becomes HIGH.

And in the case of an inside edge signal 1840 of FIG. 10D, since 2pixels are pattern HIGH and 8 pixels are pattern LOW, pattern LOW isvalid and pattern HIGH is invalid, and therefore the isolation amountdecision signal becomes LOW.

Pattern matching is carried out in this manner using patterns of aplurality of sizes. The present embodiment was described in regard toinside edge signals, but the same pattern matching is also carried outin regard to the outside edge signals. In this case, patterns anddecision level adjustment values can be set arbitrarily.

Next, the isolation amount decision signals output from the isolationamount decision units 1205 and 1206 are supplied to OR processing units1207 and 1208. At the OR processing units 1207 and 1208, an OR isobtained of the HIGH/LOW of the isolation amount decision signals withina 3×3 area, and the pixel of interest is set to HIGH if even one pixelis HIGH. A specific example is described using FIGS. 11A to 11C.

FIG. 11A shows a case where the isolation amount decision signal of onlythe pixel of interest in the center of the 3×3 area is HIGH.Accordingly, after OR processing, the pixel of interest is left as it isas HIGH. In FIG. 11B, the pixel of interest of the 3×3 area is LOW, butsince there is a HIGH pixel within the 3×3 area, the pixel of interestbecomes HIGH after OR processing. In FIG. 1C, only the pixel of interestof the 3×3 area is LOW, but since there are HIGH pixels within the 3×3area, the pixel of interest becomes HIGH after OR processing.

Signals resulting from processing by the OR processing units 1207 and1208 are supplied to accumulation processing units 1209 and 1210.Accumulation signals that have undergone accumulation processing passthrough threshold decision units 1211 and 1212 then through halftone dotdecision units 1213 and 1214 and are output as a halftone dot signal A(1215) and a halftone dot signal B (1216). This process is describedusing FIG. 12.

FIG. 12 shows a specific example of the OR processing unit 1207, theaccumulation processing unit 1209, the threshold decision unit 1211, andthe halftone dot decision unit 1213. A configuration involving the ORprocessing unit 1208, the accumulation processing unit 1210, thethreshold decision unit 1212, and the halftone dot decision unit 1214 isessentially equivalent and therefore description thereof is notincluded. It should be noted that “M×N” in parentheses after eachprocessing unit indicates an area size targeted for processing by thatprocessing unit.

Signals that have undergone OR processing by the OR processing unit 1207are accumulated using a plurality of areas. For example, an accumulationprocessing unit 2011 accumulates in a 9×9 area signals that have becomeHIGH after OR processing. Accordingly, output values of the accumulationprocessing unit 2011 are in a range of 0 to 81. This is the same for anaccumulation processing unit 2012 and an accumulation processing unit2013 (except that the areas may be of different sizes, e.g., 15×15 and21×21 in the example shown in FIG. 12).

Next, the accumulated signals from accumulation processing unit 2011,accumulation processing unit 2012 and accumulation processing unit 2013undergo corresponding processing in a threshold decision unit 2021, athreshold decision unit 2022, and a threshold decision unit 2023,respectively. These thresholds are set to a different value for eachrespective threshold decision.

Based on the signals output from the respective threshold decisionunits, a halftone dot decision unit 1213 carries out decisions as towhether or not there is a halftone dot and outputs a halftone dot signalA (1215). A more specific example is described using FIGS. 13A to 13C.

Numeral 2100 in FIG. 13A indicates OR signals in a low luminancehalftone dot area that have undergone OR processing. For black pixels itis indicated that the OR signal is HIGH and for white pixels it isindicated that the OR signal is LOW. These signals are accumulated in aplurality of different areas. Three types of areas are described here tofacilitate description, but there is no limitation to three types. Foreach of a 9×9 area 2102, a 15×15 area 2103, and a 21×21 area 2104, whichare centered on a pixel of interest 2101, the accumulation processingunits 2011, 2012, and 2013 accumulate (add) numbers of pixels whose ORsignal is HIGH within the 9×9 area, the 15×15 area, and the 21×21 area.

FIG. 13B shows these respective accumulation results. The accumulationresult of the accumulation processing unit 2011 is 12 pixels. Theaccumulation result of the accumulation processing unit 2012 is 40pixels. The accumulation result of the accumulation processing unit 2013is 76 pixels.

At this time, if it is assumed that a halftone dot is set when HIGHsignals within an area exceed 5%, and a halftone dot is not set for 5%or lower, the threshold settings to the threshold decision unit 1213 areas follows.

Threshold for the accumulation processing unit 2011 is “5,”

Threshold for the accumulation processing unit 2012 is “12,” andThreshold for the accumulation processing unit 2013 is “23.”

In the case of FIG. 13A, the accumulation result “12” of theaccumulation processing unit 2011 is above the threshold “5” andtherefore it is decided that the pixel of interest is a halftone dotarea. Similarly, the accumulation result “40” of the accumulationprocessing unit 2012 exceeds the threshold “12” and the accumulationresult “76” of the accumulation processing unit 2013 also exceeds thethreshold “23,” and therefore a decision of halftone dot is made.

Next, based on the aforementioned three types of results, a decision ismade as to whether or not the pixel of interest is to be a halftone dot.For example, as shown in FIG. 13C, pairs of two groups are formed fromthe results of the three types. For example, one group is obtained bythe accumulation results from the accumulation processing unit 2011 andthe accumulation processing unit 2012 and another group is obtained fromthe accumulation results from the accumulation processing unit 2012 andthe accumulation processing unit 2013. Then, a logical product is takenwithin the respective groups. The logical product of the accumulationresults of the accumulation processing unit 2011 and the accumulationprocessing unit 2012 decides a halftone dot. The logical product of theaccumulation results of the accumulation processing unit 2012 and theaccumulation processing unit 2013 decides a halftone dot. Further still,a logical sum is determined of the results of the two logical productsobtained in this manner. Since the results of the logical products areboth for halftone dots in the foregoing case, the logical sum is alsofor a halftone dot, and finally it is decided that the pixel of interestis in a halftone dot area.

Description is given using FIGS. 14A to 14C of a case where there is acharacter edge within a low luminance halftone dot area.

An area 2200 in FIG. 14A indicates OR signals in a low luminancehalftone dot area that have undergone OR processing. For black pixels itis indicated that the OR signal is HIGH and for white pixels it isindicated that the OR signal is LOW. When there is a character edge 2205as shown in the diagram, no isolation point occurs in the vicinity ofthe character edge and therefore the OR signals of the character edgevicinity are LOW. In this state, these signals are accumulated in aplurality of different areas. The accumulation processing unit 2011, theaccumulation processing unit 2012, and the accumulation processing unit2013 accumulate numbers of pixels whose OR signal is HIGH for each of a9×9 area 2202, a 15×15 area 2203, and a 21×21 area 2204, which arecentered on a pixel of interest 2201.

Respective accumulation results are as shown in FIG. 14B. Theaccumulation result of the accumulation processing unit 2011 is “4,” Theaccumulation result of the accumulation processing unit 2012 is “32,”and the accumulation result of the accumulation processing unit 2013 is“60.” When each of the thresholds is set having the same conditions asearlier, the accumulation value “4” of the accumulation processing unit2011 is under the threshold “5” and therefore the pixel of interest isdecided as non halftone dot. Furthermore, the accumulation value “32” ofthe accumulation processing unit 2012 is not less than the threshold“12” and therefore the pixel of interest is decided as a halftone dot.And the accumulation value “60” of the accumulation processing unit 2013is not less than the threshold “23” and therefore the pixel of interestis likewise decided as a halftone dot. FIG. 14B shows these decisionresults.

Next, as shown in FIG. 14C, based on the aforementioned three types ofdecision results, a decision is made as to whether or not the pixel ofinterest is to be a halftone dot. As before, pairs of two groups areformed from the results of the three types. A logical product isobtained for each of one group of decision results of the accumulationprocessing unit 2011 and the accumulation processing unit 2012 andanother group of decision results of the accumulation processing unit2012 and the accumulation processing unit 2013. With the logical productof decision results based on the accumulation results from theaccumulation processing unit 2011 and the accumulation processing unit2012, a decision is made of non halftone dot. With the logical productof decision results based on the accumulation results from theaccumulation processing unit 2012 and the accumulation processing unit2013, halftone dot is decided. Further still, a logical sum isdetermined of the results of the two logical products. In the foregoingcase, one decision is non halftone dot and another decision is halftonedot, and therefore the logical sum is halftone dot and the result of thehalftone dot decision is halftone dot. In this way, detection ofhalftone dots can be achieved using a plurality of areas and a pluralityof thresholds.

Next, description is given using FIGS. 15A to 15C of a case where thereis a pixel of interest outside an edge of a low luminance halftone dotarea.

An area 2300 in FIG. 15A indicates OR signals in a low luminancehalftone dot area that have undergone OR processing. For black pixels itis indicated that the OR signal is HIGH and for white pixels it isindicated that the OR signal is LOW. In this state, these signals areaccumulated in a plurality of different areas. FIG. 15B shows results ofaccumulating numbers of pixels whose OR signal is HIGH for each of a 9×9area 2302, a 15×15 area 2303, and a 21×21 area 2304, which are centeredon a pixel of interest 2301.

The accumulation result of the accumulation processing unit 2011 forarea 2302 is “0,” The accumulation result of the accumulation processingunit 2012 for area 2303 is “12,” and the accumulation result of theaccumulation processing unit 2013 for area 2304 is “20.”

When each of the thresholds is set having the same conditions asearlier, the accumulation value “0” of the accumulation processing unit2011 is under the threshold “5” and therefore the pixel of interest isdecided as non halftone dot. The accumulation value “12” of theaccumulation processing unit 2012 is equivalent to the threshold “12”and therefore the pixel of interest is decided as a halftone dot. Andthe accumulation value “20” of the accumulation processing unit 2013 isbelow the threshold “23” and therefore the pixel of interest is decidedas non halftone dot. FIG. 15B shows these decision results.

Next, based on the aforementioned three types of results, a decision ismade as to whether or not the pixel of interest is to be a halftone dot.As shown in FIG. 15C, pairs of two groups are formed from the results ofthe three types. For example, a logical product is obtained for each ofone group of decision results of accumulation values of the accumulationprocessing unit 2011 and the accumulation processing unit 2012 and foranother group of decision results of accumulation values of theaccumulation processing unit 2012 and the accumulation processing unit2013. With the logical product of the decision results of theaccumulation values of the accumulation processing unit 2011 and theaccumulation processing unit 2012, a decision is made of non halftonedot. And also with the logical product of the decision results of theaccumulation values of the accumulation processing unit 2012 and theaccumulation processing unit 2013, a decision is made of non halftonedot. Further still, a logical sum is determined of the results of thetwo logical products. In the foregoing case, both results of the logicalproducts are for non halftone dot and therefore the logical sum is alsofor no halftone dot and the result of the halftone dot decision is nonhalftone dot. As a result, there is no occurrence of fattening in thehalftone dot signal, which occurred conventionally when the accumulationarea was enlarged to improve the accuracy of the decisions. This enablesthe outside of edges of halftone dots to be detected as non halftonedots using a plurality of areas and a plurality of thresholds.

The combination of accumulation processes described here is merely oneexample and there is no limitation to this. Combinations can beconfigured freely according to purpose. Furthermore, the descriptionused logical products of two groups because three types of results ofaccumulation processes are used, but there is no limitation to this. Thenumber of inputs and the logical calculation units thereof can beconfigured freely. Moreover, the combination of logical products andlogical sums is merely one example and there is no limitation to this.Logical products and logical sums can be combined freely.

[Description of Character-in-Halftone Dot Decision Unit 1005]

FIG. 16 is a block configuration diagram of the character-in-halftonedot decision unit 1005 according to the present embodiment.

An adaptive smoothing unit 2401 receives decision signals from thedecision signal generating unit 1002 and carries out an adaptivesmoothing process. Here, a digital filtering process is carried out bywhich predetermined frequency components of the image data are smoothedwhile characters/fine lines are excluded adaptively.

Smoothing signals output from the adaptive smoothing unit 2401 undergoan edge emphasizing process at an edge emphasizing unit 2402. Here, adigital filtering process is carried out by which predeterminedfrequency components of the image data are emphasized and extracted. Atypical example of this is a secondary differential filter or the likesuch as a Laplacian.

Edge emphasized signals output from the edge emphasizing unit 2402 areinput to a threshold decision unit 2403. A positive value threshold isset in the threshold decision unit 2403.

When using a secondary differential filter, the filtered signal valueshave positive or negative signs, and these are shown in FIGS. 7A to 7Cfor a case in which the image signals are luminance signals as in thepresent embodiment. The edge boundary area 1501 shown in the diagramindicates a portion of a black character (character edge) on the whitebackground 1503. Numeral 1502 indicates the character inside area.Viewing the section 1510 of the edge boundary area 1501 as a signallevel gives the image signal 1504. The character inside area 1502 is adark area and therefore the signal level is low (1505). The background1503 is a bright area and therefore the signal level is high (1506). Theimage signal 1504 subjected to digital filtering process using asecondary differential filter becomes the edge extraction signal 1507.The character inside area 1502 side takes a positive value and thebackground 1503 side takes a negative value.

The character-in-halftone dot decision aims at extracting the characteritself within the halftone dot area and therefore a character inhalftone dot signal 2404 is obtained by deeming the character outsideedge as a halftone dot area and extracting the inside edge of thecharacter. Thus, a positive threshold is set in the threshold decisionunit 2403 and an inside edge signal is output when this positivethreshold is exceeded. The foregoing process is described in furtherdetail below.

First, using FIG. 17, specific description is given concerning theprocessing of the adaptive smoothing unit 2401 shown in FIG. 16.

FIG. 17 is a process block diagram of the adaptive smoothing unit 2401.An input signal 2501 is branched into three, and the first of these isinput to an M×M filtering unit 2502 (M is a natural number) where itundergoes filtering. Here a smoothing process is executed aimed atreducing sensitivity in a prescribed frequency band. The M×M filteringunit 2502 supplies results of the smoothing process to a selector 2504.

The second of these is input to the selector 2504 without undergoing anyprocessing. At this time, image delay occurs corresponding to the numberof processing lines of the M×M filtering unit 2502, and therefore theinput signals are supplied to the selector 2504 after being delayedusing a delay memory (not shown).

The third of these is input to a vertical line, horizontal line, andslanted line detection unit 2503. Using a process buffer of M lines ofthe M×M filtering unit 2502, detection of vertical lines, horizontallines, and slanted lines is carried out in an L×N (L and N are naturalnumbers and it is preferable that N≦M) area. Once a line portion isdetected, a 1 bit signal is generated per pixel, which is used inswitching the selector 2504.

Based on the selector switching signal outputted from the vertical line,horizontal line, and slanted line detection unit 2503, the selector 2504switches between and outputs the filtered signal outputted from the M×Mfiltering unit 2502 and the unfiltered signal. At this time, anadaptively smoothed signal 2505 is output wherein when a line portionhas been detected by the vertical line, horizontal line, and slantedline detection unit 2503, an unprocessed signal is output and when aline portion is not detected, a smoothed signal is output.

Here, description is given concerning the vertical line, horizontalline, and slanted line detection unit 2503. The vertical line,horizontal line, and slanted line detection unit 2503 detects lineportions using patterns such as those shown in FIGS. 18A to 18D. Itshould be noted that the horizontal direction in the diagram indicates amain scanning direction of the image and the vertical directionindicates a sub scanning direction. From a perspective of sharingprocessing memory it is preferable that a number N of sub scanningdirection lines (pixels) is smaller than a number M of sub scanningdirection processing lines of the M×M filtering unit 2502, but there isno limitation to this. A number L of main scanning direction pixels maybe larger than M. The present embodiment is described using an examplein which line portions are detected in an area of main scanningdirection (L) 7 pixels and sub scanning direction (N) 5 pixels.

A pattern 2600 in FIG. 18A indicates a pattern for detecting verticallines, a pattern 2610 in FIG. 18B indicates a pattern for detectinghorizontal lines, and patterns 2620 and 2630 in FIGS. 18C and 18D,respectively, indicate patterns for detecting slanted lines. Here,description is given of a vertical line detection method using thepattern 2600.

The pattern 2600 is shown as a 7×5 area and arranged within this are 1×5pixel blocks 2602, 2603, and 2604. A luminance total (ALL), a largestvalue (MAX), and a smallest value (MIN) are detected for each of theseblocks.

At this time, the following conditional expressions are evaluated:

MAX(2602)−MIN(2602)≦threshold Th1  (1)

MAX(2603)−MIN(2603)≦threshold Th1  (2)

ALL(2603)≦ALL (2602)−threshold Th2  (3)

Condition (1) evaluates whether there is no shading variation in thefive pixels within the 1×5 pixel block 2602. Condition 2 evaluateswhether there is no shading variation in the five pixels within the 1×5pixel block 2603. Condition 3 compares the luminance total of the 1×5pixel block 2602 and the 1×5 pixel block 2603 and evaluates whether aluminance level of the 1×5 pixel block 2603 is relatively lower thanthat of the 1×5 pixel block 2602. When it is lower, it is decided thatthe 1×5 pixel block 2603 constitutes a black line.

And similarly, the following conditional expressions are evaluated:

MAX(2604)−MIN(2604)≦threshold Th1  (4)

MAX(2603)−MIN(2603)≦threshold Th1  (5)

ALL(2603)≦ALL(2604)−threshold Th2  (6)

Of the above six conditions, if conditions 1, 2, and 3 are all metsimultaneously, or if conditions 4, 5, and 6 are all met simultaneously,then it is decided that the pixel of interest 2601 pertains to avertical line.

Specific examples are shown in FIGS. 19A to 19C. A pattern 2700 in FIG.19A involves a 7×5 area extracted from an image having a vertical lineon a white background. Dark portions and gray portions in the diagramshow the vertical line that has been read. Description is given assuming8-bit, 1 channel luminance signals (0 to 255) and, for convenience,black pixels being level 0, gray pixels (diagonal lines) being level128, and white pixels being level 255. A threshold 1 is set to 20 and athreshold 2 is set to 200. When the above-described condition 1 isapplied to the pattern 2700 to decide true/false, the following isobtained:

MAX(2602)−MIN(2602)≦threshold Th1 128−128≦20→true  (1)

MAX(2603)−MIN(2603)≦threshold Th1 0−0≦20→true  (2)

ALL(2603)≦ALL(2602)−threshold Th2 0×5≦128×5−200→true  (3)

MAX(2604)−MIN(2604)≦threshold Th1 128−128≦20→true  (4)

MAX(2603)−MIN(2603)≦threshold Th1 0−0≦20→true  (5)

ALL(2703)≦ALL(2704)−threshold Th2 0×5≦128×5−200→true  (6)

Condition (1), condition (2), and condition (3) are all true andtherefore a vertical line is detected. Furthermore, condition (4),condition (5), and condition (6) are also all true and therefore avertical line is detected by this decision also.

When this is applied to a pattern 2710 (horizontal line) in FIG. 19B,the true/false results of conditions 1 to 6 are as follows.

(1) false(2) false(3) false(4) false(5) false(6) false

Thus, no vertical line portion is detected.

When this is applied to a pattern 2720 (slanted line) in FIG. 19C, thetrue/false results of conditions 1 to 6 are as follows.

(1) false(2) false(3) true(4) false(5) false(6) true

Thus, here too no vertical line portion is detected.

The pixel blocks 2612, 2613 and 2614 of pattern 2610 shown in FIG. 18Bare used to detect a horizontal line. The pixel blocks 2622, 2623, 2624,2532, 2633 and 2634 of and patterns 2620 and 2630 in FIG. 18C and FIG.18D are used to detect slanted lines.

Examples are shown in FIGS. 20A to 20C. A pattern 2810 in FIG. 20A is anexample of horizontal line detection. Patterns 2820 and 2830 in FIG. 20Band FIG. 20C are examples of slanted line detection.

Depending on the thickness of the line, detection may not be possiblewith the present technique. Line portions having a certain degree ofthickness will not be detected as line portions by any of the patternsshown in FIGS. 18A to 18D, but since these are detected as line portionsas a result of the following process of the adaptive smoothing unit2401, this is not a problem. Line portions such as these can beextracted when smoothed image signals are emphasized by the edgeemphasizing unit 2402 and then undergo threshold processing by thethreshold decision unit 2403. The present technique involves a processthat aims to avoid high frequency extremely fine lines becoming anobject of a smoothing filter.

FIG. 21 shows one example of space filter frequency property 2900 usedby the adaptive smoothing unit 2401. The horizontal axis indicates aspace frequency property and the vertical axis indicates a frequencyresponse corresponding to that. The frequency response becomes smallerfor higher frequency bands and at or above a certain space frequency(2902) there is no response. Description is given using FIGS. 22A to 22Cof an example of when a character-in-halftone dot decision is carriedout involving the adaptive smoothing unit 2401 using this smoothingfilter.

An image 3000 in FIG. 22A is one portion extracted from a halftone dotimage. The output screen frequency of the halftone dots (3001) is high,and in terms of the frequency property of the filter shown in FIG. 21,these are halftone dots having a property such that their spacefrequency is in a position indicated by numeral 2903. Thus, when thehalftone dot image 3000 undergoes filtering by a filter having the spacefrequency property of FIG. 21, the periodic structure of the halftonedots is eliminated and a smoothed image 3010 is obtained.

A case in which there is a character within halftone dots of the samescreen frequency is described using FIG. 22B. A character 3021 isprinted within a halftone dot image 3020 and when adaptive smoothing2401 is executed on this image, a smoothed image 3030 is obtained. Thehalftone dot areas are smoothed and the periodic structure of thehalftone dots is eliminated, while the character areas are excluded fromsmoothing by an adaptive process such that a character area 3031 remainsclearly. When this image undergoes edge emphasizing by the edgeemphasizing unit 2402, an image 3040 shown in FIG. 22C is obtained. Thecharacter edges are emphasized. When the emphasized image undergoes athreshold decision process by the threshold decision unit 2403, acharacter-in-halftone dot image 3050 is obtained. White areas are theareas decided as a character in halftone dots. A fine character is shownin the example for the present embodiment and therefore the entirecharacter is extracted as a character in halftone dots. Although notshown in the diagrams, when the character size becomes large, ratherthan the entire character, edge portions (contours) of the character areextracted. [Description of attribute flag generating unit 1006]

The foregoing was a description of the character decision unit 1003, thehalftone dot decision unit 1004, and the character-in-halftone dotdecision unit 1005 according to the present embodiment. Next,description is given concerning the attribute flag generating unit 1006according to the present embodiment.

The attribute flag generating unit 1006 generates an attribute flag foreach pixel from the character decision results obtained by the characterdecision unit 1003, the halftone dot decision results obtained by thehalftone dot decision unit 1004, and the character-in-halftone dotdecision results obtained by the character-in-halftone dot decision unit1005. The attribute flags to be generated are determined as follows.

For halftone dot signal: HIGH & character signal: LOW→image attribute ofpixel of interest: halftone dot

For halftone dot signal: LOW & character signal: HIGH→image attribute ofpixel of interest: characterFor halftone dot signal: HIGH & character-in-halftone dot decision:HIGH→image attribute of pixel of interest: character in halftone dotsNone of the above→image attribute of pixel of interest: natural image;photo image, tonal image By making these determinations, the attributeflags are generated. Since there are four types of attribute flags asdescribed above, in the present embodiment, an attribute flag isconstituted by two bits per pixel.

Various image processing methods such as amount of edge emphasizing,color reproduction method, and image forming method and the like can becontrolled in response to the image attributes.

[Description of Print Processing]

Here, description is given concerning specific print processing takinginto account the above description.

As described earlier, image data resulting from processing by the inputimage processing unit 104 (which is stored in the image memory 105) andattribute flag data generated by the image area separation processingunit 103 (which is stored in the flag memory 106) undergo compressioncoding to be stored in the storage unit 110. Description is given hereof the print processing in accordance with the compression coded imagedata and flag data stored in the storage unit 110.

In timing with the printer engine 117 becoming available to print, thedata decompression unit 112 reads out the data stored in the storageunit 110 and carries out a decompression process (decoding process).Image data obtained by decompression is stored in the image memory 114and flag data is stored in the flag memory 115. The data decompressionunit 112 may perform decompression in accordance with the same algorithmas the data compression unit 109 and therefore description thereof isnot included.

The output image processing unit 116 generates output image data for theimage data stored in the image memory 114 based on the per-pixel flagdata stored in the flag memory 115 and outputs to the printer engine117. Consequently hereinafter description is given mainly of the outputimage processing unit 116.

FIG. 27 is a block configuration diagram mainly of the output imageprocessing unit 116.

When image data and flag data of a preset printable data amount isstored in the image memory 114 and the flag memory 115 by the datadecompression unit 112, these sets of image data and flag data aretransferred to the output image processing unit 116.

RGB image data that has been read out from the image memory 114 isconverted to recording color components of C (cyan), M (magenta), Y(yellow), and K (black) by RGB→CMYK conversion units 601 and 602. Adifference between the RGB→CMYK conversion units 601 and 602 is that theformer is a color space conversion unit specialized for character imagesand the latter carries out conversion for photo images and halftonedots.

Normally the color of characters printed in documents is black orseveral colors at the most. Consequently, in the present embodiment, theRGB CMYK conversion unit 601 converts to a closest approximate color ofpatterns of predefined colors (C, M, Y, and K). For example, whenR≈G≈B≈0, a decision may be made that the pixel of interest is black andtherefore conversion is made to a pattern of C=M=Y=0 and K=255. When theinput R, G, and B are expressed in 8 bits, an LUT can be configured inwhich the respective several upper order bits (2 bits would besufficient) are input and YMCK (each 8 bits) data is output.

On the other hand, the RGB→CMYK conversion unit 602 carries out highaccuracy RGB→YMCK conversions. These conversions are carried out usingmatrix calculations but may also be achieved by an LUT having an RGBaddress input of a total of 24 bits and 32-bit YMCK output.

A merging unit 603 merges sets of the YMCK data from the two RGB->CMYKconversion units 601 and 602 based on the attribute data from the flagmemory 115. Specifically, when the pixel of interest indicates“character,” data output from the RGB→CMYK conversion unit 601 isselected and output. And when the pixel of interest indicates “naturalimage” or “halftone dot,” data from the RGB→CMYK conversion unit 602 isselected and output. Then when the pixel of interest indicates“character in halftone dots,” sets of data from the two color spaceconversion units 601 and 602 are merged in accordance with anappropriate weighting coefficient and output. Specifically, an averagevalue of the two color conversion results is obtained.

CMYK data output from the merging unit 603 is supplied to filteringunits 604 to 606. The filtering units 604 to 606 have buffers of severallines inside and carry out two dimensional filtering. A difference amongthe filtering units 604 to 606 is that they have different coefficientsfor deciding the extent of edge emphasizing in the filtering process.

The filtering unit 604 carries out a compatible filtering process whenthe attribute of the pixel of interest is “character.” The filteringunit 605 carries out a compatible filtering process when the pixel ofinterest is for a “character in halftone dots.” And the filtering unit606 carries out a compatible filtering process when the pixel ofinterest is for a “photo image” or a “halftone dot.” The extent of edgeemphasizing in each filtering unit is: filtering unit 604>filtering unit605>filtering unit 606.

However, this is merely one example of the aforementioned extent of edgeemphasizing and the extent is not necessarily limited to this.

The above can be expressed in other words in that there is arelationship of the intensity of smoothing:

filtering unit 604<filtering unit 605<filtering unit 606.

When the attribute of the pixel of interest is “character,” a selector607 selects and outputs the data from the filtering unit 604. When thisis “character in halftone dots,” the data from the filtering unit 605 isselected and output, and when this is “photo” or “halftone dot,” thedata from the filtering unit 606 is selected and output.

Data selected and output from the selector 607 is supplied to gammacorrection units 608 and 610. The gamma correction unit 608 carries outcorrection suited to “characters” and “characters in halftone dots” andthe gamma correction unit 610 carries out correction suited to “halftonedots” and “photo images.”

In the case of characters, it is preferable that the dots formed at thetime of printing are not easily dispersed and therefore these arebinarized by an error diffusion processing unit 609. Furthermore, photoimages and halftone dots, for which tonality is viewed important,undergo a binarization process by a dithering unit 611 using a dithermatrix.

When the attribute of the pixel of interest is “character” or “characterin halftone dots,” a selector 612 selects the data from the errordiffusion processing unit 609 and outputs to the printer engine 117.When the attribute of the pixel of interest is “photo” or “halftonedot,” the selector 612 selects the data from the dithering unit 611 andoutputs to the printer engine 117.

As described above, by appropriately extracting character areas,halftone dot areas, and character areas in halftone dots, it becomespossible to implement adaptive image processing in response to theattributes thereof. Consequently, it becomes possible to reproducecharacter areas distinctly and photo areas smoothly. In particular,halftone dot areas and character areas in halftone dot documents aredistinguished and smoothing can be carried out on halftone dot areaswithout smoothing character areas and therefore excellent moire removalcan be achieved. As a result, images can be provided having high tonalreproduction qualities by performing smoothing on photo areas inhalftone dot documents to perform moire removal and then executingdithering. Furthermore, by performing an edge emphasizing process thenexecuting an error diffusion process for character areas in halftone dotdocuments, it is possible to provide crisp easy-to-read characterreproduction qualities.

It should be noted that in the present embodiment an example was shownin which an error diffusion method and a dithering method were employedas a binarizing measure, but there is no limitation to this. Forexample, when using only the error diffusion method, of the characterareas (including characters in halftone dots) and photos (includinghalftone dots), the size of the error diffusion matrix of the former canbe made smaller and the error diffusion matrix of the latter can be madebigger. Furthermore, when using only a dithering process, differentdither matrix patterns may be employed in character areas and photoareas. Also, methods other than this may be employed as binarizingmethods.

Furthermore, here an example was given of binarization, but when theprinter engine is capable of multi value recording, N-narization (pseudohalftones) may be implemented matched to a number N of gradations thatcan be recorded. For example, in the case of a laser beam printer, it ispossible to generate a pulse width modulated signal in response todarkness using a commonly known PWM (pulse width modulation) techniqueto form dots of sizes corresponding to darkness. Furthermore, with aninkjet printer that ejects ink droplets, shading can be expressed by thenumber of times an ink droplet is ejected in a same position. In thecase of a printer engine capable of expressing a single pixel in Ngradations in this manner, rather than binarization, it is possible toachieve N-narization (N≧2), and therefore there is no limitation tobinarization.

Second Embodiment

In the above-described embodiment (first embodiment), by appropriatelyextracting character areas, halftone dot areas, and character areas inhalftone dots, adaptive processing was executed in response to theattributes thereof, but in the second embodiment, an example is shown inwhich the capability for extracting characters in halftone dots iscontrolled according to the screen frequency of the halftone dots.

As described in regard to the first embodiment, there is a wide range ofhalftone dot documents whose output screen frequencies range from low tohigh. Generally, when printing is carried out in a single color using apaper in which ink is well absorbed, a low screen frequency is used. Forexample, monochrome newspapers and the like are one example and theoutput screen frequency of these is typically 85 lpi. For ordinary fullcolor printing, 175 lpi is often used, but 200 lpi may be used for photocollections and brochures. Output screen frequencies exceeding 300 lpiare used in recent high definition printing, but 150 lpi to 175 lpi isthe mainstream for full color printing.

On the other hand, an image forming method of the image processingapparatus according to the present invention aims to improve imagequality using a dithering process. Hitherto, error diffusion relatedimage forming methods that inhibit occurrences of moire have been themainstream, but in recent years dithering (screening) processes havebecome used for offering smooth, high tonal reproduction qualities.However, with dithering processes, there is a problem of interferencepatterns occurring easily, which is referred to as moire that occurs dueto interference between the periodic structure of the halftone dots ofthe document and the dithering periods. In order to avoid this, moireremoval processes are executed such as smoothing of halftone dot areas,but character areas in the halftone dots also end up being smoothed,resulting in a problem of characters becoming difficult to read.Consequently, in the first embodiment, achieving the reproductionqualities for both characters and photos was aimed at distinguishingbetween halftone dots and characters in halftone dots in halftone dotdocuments, then performing moire removal in halftone dot areas andexecuting an emphasizing process for characters in halftone dots.

However, in halftone dot documents there are halftone dot documentshaving various output screen frequencies, from low output screenfrequencies typified by monochrome newspapers to high output frequenciessuch as for brochures and photo collections. For this reason,reproduction qualities for both characters in halftone dots and photoscannot always be achieved for all documents. In particular, halftone dotedges and characters in halftone dots are sometimes decided incorrectlyin halftone dot documents having low output screen frequencies and moireremoval does not function effectively when attempting to extractcharacters in halftone dots such that moire may occur.

Furthermore, the moire removal process described in the presentembodiment is a method that uses a space filter and therefore its effectis dependent on the processing size of the filter. For example, theeffect is greater for 5×5 than for 3×3 and the effect is greater for 7×7than for 5×5. Thus, the moire removal effect of the same filter size isgreater for halftone dots of high screen frequencies than for halftonedots of low screen frequencies. Also, halftone dots have an output anglein addition to an output screen frequency and therefore a difference ineffect may occur when using a space filter for moire removal dependingon the angle.

Accordingly, a method is proposed by which high quality copy images areprovided no matter what kind of document is copied and without causingmoire.

An image area separation process that achieves this is described. FIG.23 is a block configuration diagram of this. This drawing includes itemscorresponding to FIG. 2 of the first embodiment.

An input signal 3101 is input to a decision signal generating unit 3102.For example, when the input signals are RGB signals (8 bits each), agrayscale signal (8 bits) is generated. At this time, it is possiblethat only the G channel of RGB channels is drawn out and it is alsopossible that it is determined by a calculation such as (R+2×G+B)/4 orthe like. The number of input signal channels and the number of bits arenot limited to these. Also, in regard to decision signal generatingmethods, channel numbers, and bit numbers, the foregoing was merely anexample.

The decision signals generated by the decision signal generating unit3102 are supplied to a character decision unit 3103, a halftone dotdecision unit 3104, and a character-in-halftone dot decision unit 3105,where halftone dot deciding, character deciding, andcharacter-in-halftone dot deciding are executed, respectively. At thistime, the halftone dot decision unit 3104 generates two or more halftonedot decision signals.

The decision signals generated at each decision unit 3103, 3104 and 3105undergo a comparison calculation at an attribute flag generating unit3106 and attribute flags are generated. In the second embodiment,description is given concerning a case of generating a character flag3107, a halftone dot flag 3108, a character-in-halftone dot flag 3109and a photo image flag 3110. Based on these attribute flags, optimalimage processing is executed according to characteristics of the imagecontained in the document image.

A processing configuration of the halftone dot decision unit 3104 issubstantially equivalent to FIG. 4. Specific description is givenconcerning isolation amount decision units 1205 and 1206 therein.

As described in the first embodiment, the isolation amount decision unit1205 inputs inside edge signals that are output from the thresholddecision unit 1203. Similarly, the isolation amount decision unit 1206inputs outside edge signals that are output from the threshold decisionunit 1204. The processes of the isolation amount decision unit 1205 andthe isolation amount decision unit 1206 are equivalent and thereforedescription is given here of an example of processing by the isolationamount decision unit 1205.

FIG. 24 is a block configuration diagram of the isolation amountdecision unit 1205. The isolation amount decision unit 1205 carries outa pattern matching process on inside edge signals. Halftone dotdocuments range from those whose output screen frequencies are lowscreen frequencies to those whose output screen frequencies are highscreen frequencies and therefore the sizes and intervals (distances) ofthe halftone dots vary depending on the document. For this reason,pattern matching is carried out using a plurality of patterns so as tobe able to detect halftone dots of any kind of screen frequency.

In the processing configuration shown in FIG. 24, pattern matching isexecuted with nine types of patterns 3201-3209, but there is nolimitation to this. Examples of patterns are shown in FIGS. 25A and 25B.A pattern 3300 in FIG. 25A is one example of a pattern for detectinghalftone dots of high screen frequencies. And a pattern 3310 in FIG. 25Bis one example of a pattern for detecting halftone dots of low screenfrequencies. Compared to halftone dots of high screen frequencies,halftone dots of low screen frequencies are large in size and thereforethe output size of the threshold decision also becomes large. It isnecessary to prepare a plurality of pattern sizes so as to be able todetect various sizes of halftone dots. When pattern 1 (3201) to pattern5 (3205) of FIG. 24 are set as patterns for low screen frequencies, fivetypes of larger pattern masks such as the pattern 3310 are prepared. TheOR processor 1 (1207) performs logical OR calculation of the outputsfrom the pattern 1 (3201) to pattern 5 (3205) and outputs the result ofthe calculation. When pattern 5 (3205) to pattern 9 (3209) are set aspatterns for high screen frequencies, smaller pattern masks such as thepattern 3300 are prepared. The OR processor 2 (1208) performs logical ORcalculation of the outputs from the pattern 5 (3205) to pattern 9 (3209)and outputs the result of the calculation. By preparing a plurality oftypes of mask sizes in this manner it is possible to support a pluralityof output screen frequencies of halftone dots. Furthermore, by preparingthe sizes of the pattern masks incrementally, it is possible to supportthe sizes of output screen frequencies of halftone dots. Furthermore, asshown in FIG. 24, the pattern 5 may be shared for both low screenfrequencies and high screen frequencies.

These patterns are merely one example, and pattern masks may be preparedgiving consideration to such factors as the output screen frequency, theangle, the luminance (halftone dot ratio) and the like of the halftonedots. It should be noted that a specific matching method is as describedfor the first embodiment. And the processing from the isolation amountdecision onward is as shown in FIG. 4.

By giving consideration to the output screen frequencies of halftonedots in this manner and carrying out the isolation amount decisions withpattern matching that varies according to the sizes thereof, it becomespossible to generate halftone dot signals corresponding to the outputscreen frequencies of halftone dots. Here description was given of anexample in which the halftone dot signals were divided into two types,but there is no limitation to this. There may be more types than this asnecessary. Furthermore, the output screen frequencies of halftone dotsto be detected may be completely discerned to generate the halftone dotsignal 1 (1215) and the halftone dot signal 2 (1216), but detection mayalso be performed by overlapping the output screen frequencies to bediscerned. For example, the halftone dot signal 1 may be detected for150 lpi or less and the halftone dot signal 2 may be detected for 100lpi or more.

The thus-detected two types of halftone dot signals, the characterdecision signal, and the character-in-halftone dot decision signal areinput to the attribute flag generating unit 3106 where the respectivedecision signals undergo comparison calculations to generate attributeflags. In the present second embodiment, three types of flags, namelythe character flag 3107, the halftone dot flag 3108, and thecharacter-in-halftone dot flag 3109 are generated, but description isgiven using FIG. 26 in particular concerning a generation method of acharacter-in-halftone dot flag of the character-in-halftone dot decisionunit 3105.

A character-in-halftone dot flag 3409 is generated based on a highscreen frequency halftone dot signal 3401, a low screen frequencyhalftone dot signal 3403, and a character-in-halftone dot signal 3402.The low screen frequency halftone dot signal 3403 is negated (NOT) in anegating circuit 3404 and converted to a non-low screen frequencyhalftone dot signal 3405. A logical product (AND) of the convertednon-low screen frequency halftone dot signal 3405 and thecharacter-in-halftone dot signal 3402 is obtained by an AND (logicalproduct) circuit 3406 to generate a non-low screen frequencycharacter-in-halftone dot signal 3407. A logical product (AND) of thegenerated non-low screen frequency character-in-halftone dot signal 3407and the high screen frequency halftone dot signal 3401 is obtained by anAND (logical product) circuit 3408 to generate a character-in-halftonedot flag 3409.

The thus-generated character-in-halftone dot flag 3409 means that onlycharacters in halftone dot documents whose output screen frequency is ahigh screen frequency are extracted. Characters in halftone dotdocuments whose output screen frequency is a low screen frequency arenegated in advance and therefore not extracted.

By using a configuration such as this for ordinary color documents,smooth reproduction can be achieved for halftone dot areas by performingmoire removal. Furthermore, character areas in halftone dots can bereproduced distinctly by performing an appropriate emphasizing processand it becomes possible to achieve improved image quality for bothcharacters in halftone dot documents and photos. Furthermore, forhalftone dot documents having low screen frequencies such as newspapers,smooth reproduction can be achieved without causing moire. Furthermore,reproduction of characters in halftone dots can be improved when copyingprinted materials of any screen frequency without causing moire.Furthermore, by controlling screen frequency for detection of highscreen frequency halftone dots and low screen frequency halftone dots itbecomes possible to control the output screen frequencies of halftonedots for which improving the reproduction quality of characters inhalftone dots is aimed at.

Embodiments of the present invention have been described above, but thedescription of the embodiments is only an example of the presentinvention, which is not limited to only the above description. Forexample, thresholds of “2” were set in the threshold decision units 1107and 1108, but there is no limitation to this. Furthermore, thethresholds of the threshold decision units 2021 to 2023 in FIG. 12 wereset to 5% of the surface area, but this too may be appropriatelycorrected.

Furthermore, the embodiments were described using an example ofapplication to a copier, but these may also be applied to ordinaryinformation processing apparatuses such as personal computers and thelike to which an image scanner and printer are connected. In this case,the invention is achieved mainly using an application program executedon the information processing apparatus. The application includesprogram modules (functions or subroutines) corresponding to eachprocessing unit excluding the scanner 101 and the printer engine 117 inFIG. 1, with memory being maintained from a RAM provided in theinformation processing apparatus and a hard disk or like being utilizedfor the storage unit 110.

Further still, normally a computer program is stored on acomputer-readable storage medium such as a CD-ROM. To make itexecutable, it is necessary for it to be loaded into a reading device(for a CD-ROM this is a CD-ROM drive) on the computer and copied orinstalled onto the system.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadcast interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2006-135865 filed on May 15, 2006, which is hereby incorporated byreference herein in its entirety.

1. An image processing apparatus in which an attribute of each pixelconstituting image data is decided, comprising: a generation unitadapted to generate a decision signal, having luminance as a maincomponent, from the image data; a character decision unit adapted to,based on the decision signal generated by the generation unit, generateinside edge data indicating whether a pixel of interest is positioned onan image edge of a low luminance side at which the luminance changesfrom low luminance to high luminance, outside edge data indicatingwhether the pixel of interest is positioned on an image edge of a highluminance side at which the luminance changes from high luminance to lowluminance, and accumulation data of inside edge data and outside edgedata of a predetermined number of pixels positioned around the pixel ofinterest, and decide whether the pixel of interest is in a characterarea based on the inside edge data, the outside edge data, and theaccumulation data; a halftone dot decision unit adapted to, based on thedecision signal generated by the generation unit, generate inside edgedata indicating whether the pixel of interest is positioned on an imageedge of a low luminance side at which the luminance changes from lowluminance to high luminance and outside edge data indicating whether thepixel of interest is positioned on an image edge of a high luminanceside at which the luminance changes from high luminance to lowluminance, calculate an isolation degree for each of multiple patternsby matching isolation patterns of different sizes against the insideedge data and outside edge data of a predetermined number of pixelsaround and including the pixel of interest, and decide whether the pixelof interest is in a halftone dot area based on the calculated isolationdegrees; an adaptive filter unit adapted to decide whether there iscontinuity in low luminance pixels of a preset plurality of directionsbased on the decision signal generated by the generation unit, and allowthe decision signal to pass when it is decided that there is continuityand carry out a low pass filter process defined by a preset frequencywhen it is decided that there is no continuity; a character-in-halftonedot decision unit adapted to, based on the decision signal generated bythe adaptive filter unit, generate inside edge data indicating whetherthe pixel of interest is positioned on an image edge of a low luminanceside at which the luminance changes from low luminance to high luminanceand decide whether the pixel of interest is a character area in ahalftone dot area based on the generated inside edge data; and anattribute data generation unit adapted to generate attribute data of thepixel of interest based on decision results of the character decisionunit, the halftone dot decision unit, and the character-in-halftone dotdecision unit.
 2. The image processing apparatus according to claim 1,further comprising: a print image generation unit adapted to generateprint image data based on image data and the attribute data; and aprinting unit adapted to print an image on a printing medium based onthe generated print image data, wherein the print image generation unitcomprises: a color space conversion unit adapted to adaptively convert acolor space of the image data into image data of a recording color spacein accordance with the attribute data; a filtering unit adapted tofilter the image data generated by the color space conversion unit inaccordance with the attribute data; and an N-narization unit adapted toapply n-value processing to the image data generated by the filteringunit using an N-narization method (N≧2) in accordance with the attributedata.
 3. The image processing apparatus according to claim 1, whereinthe halftone dot decision unit comprises: a counting unit adapted to setareas R1, R2, . . . of different sizes and count a number of pixels Pithat have been decided as an edge inside the areas Ri (i=1, 2, . . . )including a pixel of interest; a calculation unit adapted to calculate aratio Pi/Ri with respect to a size of each area of a number counted bythe counting unit and calculate decision data Qi indicating whether eachratio Pi/Ri exceeds a preset threshold; and a logical operation unitadapted to obtain a logical product of groups of calculated decisiondata {Qi, Q (i+1)}, {Q (i+1), Q (i+2)} . . . and obtain a logical sum ofeach logical product result, wherein the halftone dot decision unittakes a result of the logical operation unit as a decision result of thehalftone dot decision unit.
 4. The image processing apparatus accordingto claim 3, wherein the counting unit counts a number of pixels decidedas an inside edge inside the area Ri (i=1, 2, . . . ) and a number ofpixels decided as an outside edge outside the area Ri (i=1, 2, . . . ).5. A method of an image processing apparatus in which an attribute ofeach pixel constituting image data is decided, comprising: a generationstep of generating a decision signal having luminance as a maincomponent from the image data; a character decision step of, based onthe decision signal generated in the generation step, generating insideedge data indicating whether a pixel of interest is positioned on animage edge of a low luminance side at which the luminance changes fromlow luminance to high luminance, outside edge data indicating whetherthe pixel of interest is positioned on an image edge of a high luminanceside at which the luminance changes from high luminance to lowluminance, and accumulation data of inside edge data and outside edgedata of a predetermined number of pixels positioned around the pixel ofinterest, and deciding whether the pixel of interest is in a characterarea based on the inside edge data, the outside edge data, and theaccumulation data; a halftone dot decision step of, based on thedecision signal generated by the generation step, generating inside edgedata indicating whether the pixel of interest is positioned on an imageedge of a low luminance side at which the luminance changes from lowluminance to high luminance and outside edge data indicating whether thepixel of interest is positioned on an image edge of a high luminanceside at which the luminance changes from high luminance to lowluminance, calculating an isolation degree for each of multiple patternsby matching isolation patterns of different sizes against the insideedge data and outside edge data of a predetermined number of pixelsaround and including the pixel of interest, and deciding whether thepixel of interest is in a halftone dot area based on the calculatedisolation degrees; an adaptive filter step of deciding whether there iscontinuity in low luminance pixels of a preset plurality of directionsbased on the decision signal generated in the generation step, andallowing the decision signal to pass when it is decided that there iscontinuity and carrying out a low pass filter process defined by apreset frequency when it is decided that there is no continuity; acharacter-in-halftone dot decision step of, based on the decision signalgenerated in the adaptive filter step, generating inside edge dataindicating whether the pixel of interest is positioned on an image edgeof a low luminance side at which the luminance changes from lowluminance to high luminance and deciding whether the pixel of interestis a character area in a halftone dot area based on the generated insideedge data; and an attribute data generation step of generating attributedata of the pixel of interest based on decision results of the characterdecision step, the halftone dot decision step, and thecharacter-in-halftone dot decision step.
 6. A computer program stored ona computer-readable storage medium that, by being read in and executedby a computer, functions as an image processing apparatus in which anattribute of each pixel constituting image data is decided, the computerprogram functioning as: a generation unit adapted to generate a decisionsignal having luminance as a main component from the image data; acharacter decision unit adapted to, based on the decision signalgenerated by the generation unit, generate inside edge data indicatingwhether a pixel of interest is positioned on an image edge of a lowluminance side at which the luminance changes from low luminance to highluminance, outside edge data indicating whether the pixel of interest ispositioned on an image edge of a high luminance side at which theluminance changes from high luminance to low luminance, and accumulationdata of inside edge data and outside edge data of a predetermined numberof pixels positioned around the pixel of interest, and decide whetherthe pixel of interest is in a character area based on the inside edgedata, the outside edge data, and the accumulation data; a halftone dotdecision unit adapted to, based on the decision signal generated by thegeneration unit, generate inside edge data indicating whether the pixelof interest is positioned on an image edge of a low luminance side atwhich the luminance changes from low luminance to high luminance andoutside edge data indicating whether the pixel of interest is positionedon an image edge of a high luminance side at which the luminance changesfrom high luminance to low luminance, calculate an isolation degree foreach of multiple patterns by matching isolation patterns of differentsizes against the inside edge data and outside edge data of apredetermined number of pixels around and including the pixel ofinterest, and decide whether the pixel of interest is in a halftone dotarea based on the calculated isolation degrees; an adaptive filter unitadapted to decide whether there is continuity in low luminance pixels ofa preset plurality of directions based on the decision signal generatedby the generation unit, and allow the decision signal to pass when it isdecided that there is continuity and carry out a low pass filter processdefined by a preset frequency when it is decided that there is nocontinuity; a character-in-halftone dot decision unit adapted to, basedon the decision signal generated by the adaptive filter unit, generateinside edge data indicating whether the pixel of interest is positionedon an image edge of a low luminance side at which the luminance changesfrom low luminance to high luminance and decide whether the pixel ofinterest is a character area in a halftone dot area based on thegenerated inside edge data; and an attribute data generation unitadapted to generate attribute data of the pixel of interest based ondecision results of the character decision unit, the halftone dotdecision unit, and the character-in-halftone dot decision unit.
 7. Acomputer-readable storage medium storing a computer program of claim 6.8. An image processing apparatus in which an attribute of each pixelconstituting image data is decided, comprising: a generation unitadapted to generate a decision signal having luminance as a maincomponent from the image data; an adaptive filter unit adapted to decidewhether there is continuity in low luminance pixels of a presetplurality of directions based on the decision signal generated by thegeneration unit, and allow the decision signal to pass when it isdecided that there is continuity and carry out a low pass filter processdefined by a preset frequency when it is decided that there is nocontinuity; and a character-in-halftone dot decision unit adapted to,based on the decision signal generated by the adaptive filter unit,generate inside edge data indicating whether the pixel of interest ispositioned on an image edge of a low luminance side at which theluminance changes from low luminance to high luminance and decidewhether the pixel of interest is a character area in a halftone dot areabased on the generated inside edge data.
 9. The image processingapparatus according to claim 8, further comprising: a detection unitadapted to detect a first halftone dot signal and a second halftone dotsignal having a halftone dot output screen frequency lower than that ofthe first halftone dot signal; and a flag generating unit adapted togenerate a flag indicating a character in halftone dots based on adetection result of the detection unit and a decision result of thecharacter-in-halftone dot decision unit.
 10. The image processingapparatus according to claim 9, wherein the flag generating unitgenerates a flag indicating the character in halftone dots by negatingthe second halftone dot signal to generate a second non halftone dotsignal, obtaining a logical product of the second non halftone dotsignal and a character in halftone dot signal decided as a character inhalftone dots by the character-in-halftone dot decision unit, so as togenerate a non-low screen frequency character-in-halftone dot signal andobtaining a logical product of the generated non-low screen frequencycharacter-in-halftone dot signal and the first halftone dot signal. 11.A method of an image processing apparatus in which an attribute of eachpixel constituting image data is decided, comprising: a generation stepof generating a decision signal having luminance as a main componentfrom the image data; an adaptive filter step of deciding whether thereis continuity in low luminance pixels of a preset plurality ofdirections based on the decision signal generated in the generationstep, and allowing the decision signal to pass when it is decided thatthere is continuity and carrying out a low pass filter process definedby a preset frequency when it is decided that there is no continuity;and a character-in-halftone dot decision step of, based on decisionsignals generated in the adaptive filter step, generating inside edgedata indicating whether a pixel of interest is positioned on an imageedge of a low luminance side at which the luminance changes from lowluminance to high luminance and deciding whether the pixel of interestis a character area in a halftone dot area based on the generated insideedge data.
 12. The method according to claim 11, further comprising: adetection step of detecting a first halftone dot signal and a secondhalftone dot signal having a halftone dot output screen frequency lowerthan that of the first halftone dot signal; and a flag generating stepof generating a flag indicating a character in halftone dots based on adetection result of the detection step and a decision result of thecharacter-in-halftone dot decision step.
 13. The method according toclaim 12, wherein the flag generating step includes generating a flagindicating the character in halftone dots by negating the secondhalftone dot signal to generate a second non halftone dot signal,obtaining a logical product of the second non halftone dot signal and acharacter in halftone dot signal decided as a character in halftone dotsin the character-in-halftone dot decision step so as to generate anon-low screen frequency character-in-halftone dot signal, and obtaininga logical product of the generated non-low screen frequencycharacter-in-halftone dot signal and the first halftone dot signal. 14.A computer program stored on a computer-readable storage medium that bybeing read in and executed by a computer functions as an imageprocessing apparatus in which an attribute of each pixel constitutingimage data is decided, the computer program functioning as: a generationunit adapted to generate a decision signal having luminance as a maincomponent from the image data; an adaptive filter unit adapted to decidewhether there is continuity in low luminance pixels of a presetplurality of directions based on the decision signal generated by thegeneration unit and allow the decision signal to pass when it is decidedthat there is continuity and carry out a low pass filter process definedby a preset frequency when it is decided that there is no continuity;and a character-in-halftone dot decision unit adapted to, based on thedecision signal generated by the adaptive filter unit, generate insideedge data indicating whether a pixel of interest is positioned on animage edge of a low luminance side at which the luminance changes fromlow luminance to high luminance and decide whether the pixel of interestis a character area in a halftone dot area based on the generated insideedge data.
 15. The computer program according to claim 14, furtherfunctioning as: a detection unit adapted to detect a first halftone dotsignal and a second halftone dot signal having a halftone dot outputscreen frequency lower than that of the first halftone dot signal; and aflag generating unit adapted to generate a flag indicating a characterin halftone dots based on a detection result of the detection unit and adecision result of the character-in-halftone dot decision unit.
 16. Thecomputer program according to claim 15, wherein the flag generating unitgenerates a flag indicating the character in halftone dots by negatingthe second halftone dot signal to generate a second non halftone dotsignal, obtaining a logical product of the second non halftone dotsignal and a character in halftone dot signal decided as a character inhalftone dots by the character-in-halftone dot decision unit, so as togenerate a non-low screen frequency character-in-halftone dot signal andobtaining a logical product of the generated non-low screen frequencycharacter-in-halftone dot signal and the first halftone dot signal. 17.A computer-readable storage medium storing a computer program of claim14.